1. Field of the Invention
The present invention relates generally to transmission of data frames within a satellite based cable system. More particularly, the present invention relates to synchronizing timing between components included within the satellite based cable system.
2. Related Art
Traditional cable systems transmit data in accordance with DOCSIS cable system standards. A common frame structure used within these DOCSIS based cable systems is the motion pictures expert group (MPEG) frame structure. Although the DOCSIS standards traditionally pertain to non-satellite based cable systems, various aspects and modifications of the DOCSIS standards now apply to both the upstream and downstream channels of satellite based cable systems. One such modification is known as adaptive modulation.
As known in the art, adaptive modulation is a signal processing technique used for downstream channels on traditional satellite based cable systems to maximize data rates. Adaptive modulation enables modulation schemes to adapt to changing transmission channel characteristics on a frame by frame basis. In one implementation, for example, adaptive modulation provides an ability to change physical downstream parameters, particularly along MPEG frame boundaries.
Satellite based cable systems typically use a satellite modem termination system (SMTS) to transmit data frames via a satellite network to cable subscribers. Each cable subscriber has a modem (subscriber modem) specifically configured to receive specific frames from among a stream of transmitted MPEG data frames. Adaptive modulation enables the SMTS to send the specific data frames within the stream to the different subscriber modems based on that modem's ability to receive data. Thus, within a transmitted data stream including consecutive data frames, the first frame may go to a first subscriber modem and the second data frame may go to a second subscriber modem. In such a transmission scenario, in order for the SMTS to use the most efficient transmission parameters and optimize the use of available data width, MPEG frame timing is absolutely essential.
In order to optimize the operational speed and efficiency of satellite based cable systems, an appropriate adaptive modulation scheme should be selected. The appropriate scheme will enable the SMTS to send the MPEG to the intended subscriber modem. Several components within the traditional SMTS are particularly relevant to this process. These relevant components include a media access control (MAC) chip for performing media access control and packet processing of internet protocols (IPs) embedded within the MPEG data frame. Also included is a MAC clock, for time stamping the MPEG data frames, and a modulator.
A traditional MAC chip time stamps received data frames and stores this timing data in a time stamp register, usually within the MAC chip. The time-stamped frames are then forwarded to the modulator and subsequently transmitted via satellite to one or more subscriber modems. Prior to transmission, each time stamped data frame is provided to the modulator, where each data frame is modulated based upon predetermined transmission characteristics.
Since different modulation techniques are applied (adaptively) to different data frames, each technique creates slight timing skews between the time stamp value and the time the frame is actually transmitted. These skews result from differences and complexities of the various adaptive modulation techniques that are applied. Additionally, the severity of these timing skews is exacerbated by atmospheric and other delays created during actual transmission.
While some timing errors can be compensated for within the subscriber modems, skews resulting from application of the adaptive modulation techniques cannot be as easily resolved. Additional timing errors are created due to drift between the MAC clocks and clocks within the modulator.
What is needed, therefore, is a technique to resolve downstream transmission timing errors in data frames that are adaptively modulated for transmission within a satellite based cable system. What is also needed, is a technique and mechanism to minimize drift between the clocks of, for example, the MAC chip and the modulator within the SMTS.